1. Field of the Invention
The invention relates to a signal receiving device comprising control means for switching the device between an operating state and a calibrating state, the device comprising
a tunable circuit arranged between a main signal input and a main signal output, for passing operating signals during the operating state, the device also comprising PA1 a calibrating signal source, for supplying a calibrating signal to the main signal input, the main signal output being coupled to PA1 tuning signal selecting and storing means, arranged for PA1 supplying a reference signal to the signal input, PA1 subsequently measuring the tuning signal which tunes the tunable circuit to said reference signal, PA1 storing that timing signal.
selecting a tuning signal which, when supplied to a tuning input of the tunable circuit, tunes the tunable circuit to a spectral component of the calibrating signal during the calibrating state PA2 and storing said tuning signal in memory means, which are arranged for supplying said tuning signal subsequently to the tuning input during the operating state.
The invention also relates to a method for calibrating a tunable circuit for operation under control of a stored tuning signal, the method comprising
Tunable circuits are often used in devices such as radio receivers, for example at the intermediate frequency demodulation stage. For robust and sensitive signal reception, the tunable circuit must be tuned to, or at least be tuned close to, the frequency of the signal to be received. Proper tuning can however not be guaranteed merely by the manufacture of such circuits. This is because of process fluctuations during manufacturing, and also for example due to aging and temperature effects during use.
2. Description of the Related Art
To eliminate such effects, the tuning of the tunable circuit must be calibrated. An example of a tunable circuit provided with tuning means according to the preamble is an automatic calibration system for a voltage controlled oscillator known from U.S. Pat. No. 4,847,569. This publication discloses a voltage controlled oscillator which is automatically calibrated in the calibrating state. The generator comprises a phase locked loop (PLL). In the calibrating state, the PLL is made to lock onto a reference signal which has an accurately known frequency (obtained from a crystal oscillator). The control signal for the oscillator in the locked state of the loop is then stored in the storing means. In the operating state, this stored tuning signal is supplied to the oscillator and serves to pretune the oscillator.
The known device has the disadvantage that it requires a reference signal with accurately known frequency for calibration. One way of supplying such a reference signal is to use a crystal oscillator as pan of the device, but this adds to the complexity of the device and is expensive. Another way is to use an external signal source, but this is impractical, especially when calibration is required repeatedly.